docs: adding affilaitions api (CM-1042)

[ulemons]

Note

Low Risk
Documentation-only change adding an OpenAPI contract; no runtime code paths are modified.

Overview
Adds a new OpenAPI 3.1 spec (backend/src/api/public/v1/affiliations/openapi.yaml) documenting the public Affiliations API, including POST /affiliations (bulk lookup with pagination) and GET /affiliations/{githubHandle} (single lookup).

Defines the request/response schemas (Contributor, AffiliationPeriod, BulkAffiliationsResponse) plus standard error responses and bearer API-key security requirements.

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Pull request overview

Adds an OpenAPI specification for the Public v1 “Affiliations” endpoints to document bulk and single-handle contributor affiliation lookups.

Changes:

• Introduces a new openapi.yaml describing POST /affiliations (bulk lookup with pagination) and GET /affiliations/{githubHandle} (single lookup).
• Defines request/response schemas and common error responses (400/401/403/404/429) for the Affiliations API.

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